The invention is directed to an diffractive optical structure with a high angular dispersion and low stray light, and more particularly to a grating structure with a polarization-independent diffraction efficiency that can be applied in WDM applications.
Fiber optic telecommunication systems are increasingly used to provide high-bandwidth transmission of information to homes and businesses. During the past decade, optical networks have become more complex and use multiple wavelengths transmitted simultaneously over the same fiber. This transmission method is referred to as wavelength division multiplexing/demultiplexing (WDM/D). The international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing (xcx9c0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less. In addition, the total wavelength range over which these devices are designed to operate is increasing. WDM is a general term applied to the separation and integration of information carried on these optical channels. The technologies involved in WDM/D require efficient handling of the optical signals propagating over fiber optic cables, and being routed through various devices that deliver the high bandwidth signals to the end customer.
To extract information from WDM channels, the various optical carrier frequencies propagating, for example, in a communication fiber, have to be separated. Wavelength-selective optical elements, such as interference filters, fiber Bragg gratings, arrayed waveguide gratings (AWG), and free space gratings, e.g., surface relief diffraction gratings, are employed for this purpose. Many of these wavelength-selective components have a polarization-sensitive response. The free-space gratings typically operates in a Littrow configuration. The gratings can be ruled gratings, holographic gratings or etched gratings. Etched gratings typically employ a crystalline, preferably a semiconductor substrate, that exposes crystal planes with a predetermined crystal orientation upon etching. For example, the (100) crystal planes in Si have a preferential etch rate of 400:1 over the Si (111) planes when etched with an aqueous solution of KOH.
It is well known that standard single-mode fiber may not preserve the launched state of polarization (SOP) of optical signals propagating through the fiber. Moreover, the SOP usually varies with time at any given point along the fiber due to small changes in the physical environment of the fiber or in the optical wavelengths. These random polarization fluctuations can affect transmission systems that employ polarization-sensitive optical components, such as optical amplifiers, coherent optical receivers or polarization-dependent photonic switches and demultiplexers. Polarization scramblers have recently been employed in optically amplified transoceanic communication systems, where they are used, for example, to eliminate anisotropic gain saturation (polarization hole burning) in the optical amplifiers by depolarizing the launched optical signal. Accordingly, optical components used with optical fibers should be made polarization independent, thereby reducing costs and complexity of the fiber-optic communications system.
It would therefore be desirable to provide compact wavelength-dispersive devices that can separate closely-spaced optical channels with equal efficiency regardless of the polarization direction of the light signal and with low crosstalk between channels.
The invention is directed to a grating structure for an optical multiplexer/demultiplexer which provides a substantially polarization-independent diffraction efficiency over a predetermined useful wavelength range. The diffraction grating has a surface relief structure with a repeating pattern consisting of triangular sections separated by flat sections. The structure can be used in both Littrow and Littman-Metcalf configuration.
According to one aspect of the invention, the optical multiplexer/demultiplexer having the grating structure according to the invention operates in xe2x80x9cdouble-passxe2x80x9d geometry (Littman-Metcalf configuration) to provide a greater angular dispersion and a finer channel spacing, allowing a larger number of optical wavelength channels to operate over a fiber optic network. In addition, with this structure, the received signal is cleaner as a result of reduced contribution from stray light (reduced channel cross talk).
The optical multiplexer/demultiplexer includes a prism having an input face, an output face and a grating face, with a grating being formed either directly on or alternatively applied to the grating face. A reflecting surface is disposed substantially parallel to the output face. Incident optical radiation with a predetermined wavelength or wavelength range passes through the input face and is diffracted by the grating towards the output face. The reflecting surface retro-reflects the diffracted optical radiation back onto the prism for an additional diffraction by the grating. The twice diffracted light exits the input face of the prism at a wavelength-dependent angle relative to a surface normal of the input face.
According to another aspect of the invention, optical multiplexer/demultiplexer device for applications in fiber-optic communications includes at least one input optical fiber or waveguide, at least one output optical fiber or waveguide, and a prism with an input face, an output face and a grating face. Optical radiation with a predetermined wavelength or wavelength range emanating from the input optical fiber or waveguide can be collimated by an optical element, for example a lens, before impinging on the input face of the prism. A grating is formed either directly on or alternatively applied to the grating face. A reflecting surface is disposed substantially parallel to the output face. The incident optical radiation passes through the input face and is diffracted by the grating towards the output face. The reflecting surface retro-reflects the diffracted optical radiation back onto the prism for an additional diffraction by the grating. The twice diffracted light exits the input face of the prism at a wavelength-dependent angle relative to a surface normal of the input face.
The grating can be formed as an immersion grating in or on a material that is substantially transparent over the wavelength range of interest, such as glass, Si, Ge and the like. In addition, polarization rotation means, for example a quarter waveplate or a Faraday rotator, can be placed between the reflecting surface and the output face. The grating can be etched into silicon, wherein the natural etch planes of crystalline silicon, such as the (111 ) planes, form the sides of the triangles of the grating relief structure. The flat-topped region has a width between 20% and 50%, preferable between 25% and 35% of the grating period.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments.